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<head>
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    <title>Document</title>
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        div{
            width: 100px;
            height: 100px;
            background-color: pink;
            position: absolute;
        }
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<body>
    <div></div>
    <script>
        function fn(a,b,callback){
            console.log(a + b);
            callback && callback();
        }

        fn(1,2,function(){
            console.log('我是最后调用的');
        });

        $('div').animate({
            left:500,
            bottom:100,
        },function(){
            $(this).css('backgroundColor','purple');
        });
    </script>
</body>
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